1. Field of the Invention
The invention relates generally to fuel cells for use in marine sediment and seawater.
2. Description of the Prior Art
Typically, marine/oceanographic sensors and instruments deployed in water are powered by batteries. The key limitation of battery-based power supplies is depletion (i.e., exhaustion of energy content) which limits the period of time over which a sensor or instrument can operate. Many marine/oceanographic sensors and instruments deployed in water operate for short periods of time that are easily sustained by batteries. Many others (present or envisioned) operate unattended for longer-periods of time that are fundamentally limited by battery depletion requiring costly and resource intensive retrieval and redeployment cycles. It is widely recognized that many of these sensors and instruments would provide greater benefit if they could operate persistently.
U.S. patent application Ser. No. 10/148,104, incorporated herein by reference, discloses a fuel cell. It consists of an electrode imbedded in marine sediment connected by an external electrical circuit to an electrode positioned in overlying water. This fuel cell can generate power that is indefinitely sustained by continuous flux of reactants to each electrode. In the case of the anode, the flux of reactants is primarily attributed to diffusion of the reactants through sediment porewater. In the case of the cathode, the flux of reactants is primarily attributed to motion of the water by flow/and or convection. When the anode and cathode are the same size, the amount of sustained power generated is limited by the flux of anode reactants (porewater reductants). Initially, before generation of power, the concentration of anode reactants in sediment porewater at the anode surface is the same as that in bulk sediment in vicinity of the anode. Once power generation begins by connecting the anode to the cathode through the external circuit, anode reactants are consumed at the anode surface. This creates a local depletion zone of anode reactants about the anode surface that grows in thickness over time. Because the rate of diffusion of anode reactants from the bulk sediment in vicinity of anode to the anode surface is inversely proportional to the thickness of the depletion zone, power decreases overtime. The depletion zone eventually reaches a finite thickness due to multiple processes (e.g., convection) that replenish sediment anode reactants in vicinity of anode, resulting in sustained power that is, on average invariant over time.
The magnitude of sustained power generated scales proportionally with projected area of the anode. This property arises from the fact that sustained power is limited by diffusion of anode reactants and, and in accordance with Fick's Laws of Diffusion, the net rate of reactants diffusing to an electrode surface once an appreciable depletion zone is established is proportional to the projected area of the electrode. When the electrode consists of a square plate of nominal thickness with length of side S, the projected area is determined by S×S. The amount of sustained power generated is therefore independent of anode roughness or presence of spikes, grooves, holes, or other features that increase the total surface area of the anode relative to its projected area because these features do not increase the net flux of anode reactants to the anode. It is expected that the amount of sustained power generated would be same whether the anode were inserted into sediment parallel or perpendicular to the sediment surface.
The prior fuel cell may indefinitely generate sustained power of 0.05 to 0.035 W per square meter of anode projected area depending upon the specific marine environment. Many battery-powered marine sensors and instruments require up to 5-Watt sustained power averaged over time (taking into account duty cycles of energy consuming components). The prior fuel cell would therefore need to utilize an anode with a projected area of many hundreds of square meters to be utilized with the highest power consuming battery-powered marine sensors and instruments. Such anodes would be cost prohibitive and impractical to configure and deploy with sensors and instruments and to imbed into sediment. The prior fuel cell is therefore limited to utilization of low-power consuming sensors and instruments requiring, for example, up to 0.1-Watt sustained average power.